The SSBN (Trident subs with nuclear warheads) are undergoing changes -- since the prime mission of the Trident is supposed to be over.
2) What do SSN and SSBN stand for?
Technically, "SSN" stands for submersible ship (nuclear). Now it is commonly referred to as "submarine (nuclear propulsion)", and this is the designator for fast attack (Los Angeles-class) submarines.
Similarly, "SSBN" is submersible ship (ballistic nuclear), otherwise referred to as "ballistic missile submarine (nuclear propulsion)". This class can also be called by other nicknames- Tridents or Ohio-class (based at Kings Bay, Georgia and Bangor, WA), boomers or fleet ballistic missile submarines.
3) What is the difference between a Fast Attack and Trident submarine?
The most noticeable difference is size. The Fast Attack subs are about 360ft long, with a beam (width) of about 33 ft. Trident submarines are about 560 ft long (length of two football fields!), with a beam of about 42 ft. Along with this, a Fast Attack generally has a crew of about 130 men, doing one long deployment of several months about once a year as well as many
short away times of one to three weeks a month. Tridents have two crews of about 160 men that rotate through deployments (called patrols) of about 100 days apiece, with each crew being at sea about twice a year.
Both classes still depend on secrecy and silence for operations. A Trident's main mission is carry the Trident class missiles as a deterrent, all the while hiding from the enemy. The Fast Attacks may carry Tomahawk or Harpoon missiles, with their mission being to find and track enemy subs.
http://www.submarinewivesclub.org/generalfaqs.html